mTOR has a developmental stage-specific role in mitochondrial fitness independent of conventional mTORC1 and mTORC2 and the kinase activity

The mammalian target of rapamycin (mTOR), present in mTOR complex 1 (mTORC1) and mTORC2, is a serine/threonine kinase that integrates nutrients, growth factors, and cellular energy status to control protein synthesis, cell growth, survival and metabolism. However, it remains elusive whether mTOR plays a developmental stage-specific role in tissue development and whether mTOR can function independent of its complexes and kinase activity. In this study, by inducible genetic manipulation approach, we investigated the role of mTOR and its dependence on mTOR complexes and kinase activity in mitochondrial fitness of early, progenitor stage (lineage-negative; Lin(-)) versus later, lineage-committed stage (lineage-positive; Lin(+)) of hematopoietic cells. We found that oxidative phosphorylation (OXPHOS), ATP production and mitochondrial DNA synthesis were decreased in mTOR(-/-) Lin(-) cells but increased in mTOR(-/-) Lin(+) cells, suggesting that mTOR plays a developmental stage-specific role in OXPHOS, ATP production and mitochondrial DNA synthesis. In contrast to mTOR deletion, simultaneous deletion of Raptor, a key component of mTORC1, and Rictor, a key component of mTORC2, led to increased mitochondrial DNA in Lin(-) cells and decreased mitochondrial DNA and ATP production in Lin(+) cells, suggesting that mTOR regulates mitochondrial DNA synthesis in Lin(-) and Lin(+) cells and ATP production in Lin(+) cells independent of mTORC1 and mTORC2. Similar to mTOR deletion, deletion of Raptor alone attenuated glycolysis and increased mitochondrial mass and mitochondrial membrane potential in Lin(-) cells and increased mitochondrial mass and OXPHOS in Lin(+) cells, whereas deletion of Rictor alone had no effect on these mitochondrial parameters in Lin(-) and Lin(+) cells, suggesting that mTOR regulates glycolysis and mitochondrial membrane potential in Lin(-) cells, OXPHOS in Lin(+) cells, and mitochondrial mass in both Lin(-) and Lin(+) cells dependent on mTORC1, but not mTORC2. Either Raptor deficiency or Rictor deficiency recapitulated mTOR deletion in decreasing OXPHOS in Lin(-) cells and glycolysis in Lin(+) cells, suggesting that mTOR regulates OXPHOS in Lin(-) cells and glycolysis in Lin(+) cells dependent on both mTORC1 and mTORC2. Finally, mice harboring a mTOR kinase dead D2338A knock-in mutant showed decreased glycolysis in Lin(+) cells, as seen in mTOR(-/-) Lin(+) cells, but no change in glycolysis in Lin(-) cells, in contrast to the decreased glycolysis in mTOR(-/-) Lin(-) cells, suggesting that mTOR regulates glycolysis in Lin(+) cells dependent on its kinase activity, whereas mTOR regulates glycolysis in Lin(-) cells independent of its kinase activity.